The present invention relates to an imaging lens system for use in cameras such as an SLR camera. More particularly, the present invention relates to a medium telephoto lens system that has a large aperture ratio i.e., high speed property and a comparatively wide viewing angle and which performs focusing by moving some of the lenses in the system.
The imaging lens system used in recent versions of SLR cameras is required to have not only high optical performance but also a large aperture ratio and ease and rapidity with which focusing can be performed.
High-performance medium telephoto lens systems having a large aperture ratio and a wide viewing angle are described in Unexamined Published Japanese Patent Application Nos. 48723/1984, 205625/1988, etc.
As an increasing number of modern cameras are provided with an autofocusing capability, "inner focus" and "rear focus" type lens systems have been developed to realize focusing by moving only some of the lenses in the lens system. For example, Unexamined Published Japanese Patent Application Nos. 154111/1989 and 154112/1989 proposed an "inner focus" version having a focusing lens group in a Gauss type objective, and Unexamined Published Japanese Patent Application No. 78208/1989 proposed a "rear focus" version in which the rear group of a Gauss type objective is moved for focusing.
However, if an "inner focus" system is incorporated in a telephoto lens having a large aperture ratio and a comparatively wide viewing angle, the need to provide a space for allowing the focusing lens group to move within the lens system increases the overall lens length. Furthermore, the lens diameter has to be increased in order to admit the necessary amount of marginal rays of light. As a result, the overall lens system becomes bulky.
In contrast, a "rear focus" version in which the rear group of a Gauss type objective is moved for focusing has the advantage of relative compactness but, on the other hand, it has been difficult to realize a high-performance system since substantial aberrational variations occur upon movement of the focusing group.
The present invention has been accomplished in order to solve the aforementioned problems of the prior art and its principal object is to provide a large aperture medium telephoto lens system that performs focusing by moving only the rear lens group, that is compact and that yet has high performance in that it experiences smaller amounts of aberrational variations as a result of focusing operation.